Non-indigenous, nuisance and/or harmful species of aquatic micro-organisms are discharged into territorial waters by means of ballast water carried by ships from foreign ports. These micro-organisms are predominantly aerobic organisms that require oxygen throughout their life cycle, (i.e., first as larvae, then as plankton, finally as a mature organism). These aerobic micro-organisms obtain the necessary oxygen from “dissolved” atmospheric air present in the seawater contained in a vessel's ballast tanks. Dissolved air in seawater contains all the constituent gases present in the earth's atmosphere (including oxygen) that are not chemically bonded to the atomic elements of water. Therefore, the dissolved oxygen in seawater is freely available to sustain aquatic life.
In an effort to eliminate dissolved oxygen in a vessel's ballast water, U.S. Pat. No. 6,722,933 discloses a closed loop system that depressurizes a ballast tank's ullage space while simultaneously introducing oxygen-depleted gas into the ballast water. However, this system requires expensive modifications to the ballast tank.